Two layer resist system

ABSTRACT

A resist mark comprising two layers of resist, one of which is saturated with a diluant which does not dissolve the other. In one embodiment, the two layers of resist are applied upon a substrate, the first layer of which is more soluble in a developer. The second layer is said saturated resist and the first layer is non-saturated. This composite is preferably used to form a relief mask with recessed sidewalls used in lift-off processes.

This is a division of application Ser. No. 865,814 filed Dec. 30, 1977now U.S. Pat. No. 4,180,604.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates generally to resist layer structures and moreparticularly to high sensitivity resist layers useful in lithographicprocesses.

Optical or electron beam lithography is still used as the main tool inmicro-circuit fabrication. It is expected to continue into the nearfuture.

The additive metallization technique known as "lift-off" was developedwith the advent of electron beam lithography in the late 1960's. Itprovides the metallization after the exposure and development of theresist. Basically, the lift-off technique utilizes the fact thatelectron scattering in the resist and back scattering from the substratecreates a tear-shaped energy absorption profile in the resist, whichresults in an undercut profile after resist development. In this waymetal which is evaporated over the entire surface exhibitsdiscontinuities between the metal on the substrate and the metal overthe resist. During resist removal in a suitable solvent the metal overthe resist is also removed and a clean and faithful reproduction of theimage is obtained in metal. An additional advantage of the lift-offtechnique is that multi-level metal structures can be formed because anymaterial or combination of materials that can be evaporated can be used.

One of the principal reasons for the success of the lift-off process inelectron beam lithography is the fact that the energy absorption in theresist film during exposure is not linear but reaches a maximum in abouttwo-thirds of the beam penetration range. Thus, with proper exposure anddevelopment, undercut resist profiles are easily obtainable.

In the optical exposure of photoresist, however, energy absorption ishighest at the top of the resist film and lowest at the interfacebetween the resist and the substrate due to the attenuation of the lightin the resist. Moreover, standing waves created by the light reflectedfrom the substrate are a further complication. These exposure conditionsmake it impossible to obtain undercut or even vertical resist profileswith normal UV exposure of AZ type positive photoresist.

Another technique which has been used successfully in electron beamlithography to increase resist sensitivity, while maintaining theundercut feature of the developed resist, comprises the coating of twoor more resist layers having widely different solubilities. Afterelectron beam exposure, a developer is chosen which develops the toplayer at least ten times slower than the bottom layer. Alternatively,two mutually exclusive developers can be used for the successivedevelopment of the two layers. Both of these approaches result in resistprofiles suitable for lift-off metallization.

With resists which can be used with both electron beam as well asoptical exposure systems, however, it is difficult to spin-coat twodistinct layers without excessive interaction at the interface, due tothe low prebake temperature allowed for these resists between coatings.Electron resists such as polymethylmethacrylate (PMMA) can be baked atup to 170° without deterioration; however, an AZ-type resist which issuitable for both optical and electron beam exposure cannot be bakedabove about 100° C. At this low temperature the resist film retains itssolubility in various solvents and can therefore be easily dissolved bythe application of the second layer.

Therefore, based on results with E-beam type resists, it has been wellknown that the application of two layers of standard photoresist, ofwhich the first layer is more soluble in a developer, could be used toform a relief mask with recessed sidewalls for a lift-off process. Thistype of structure could be made in several ways: (1) by reducing thesensitizer concentration in the lower part; (2) by use of a resin havinga higher molecular weight than the lower part of the resist structurewhich develops faster than the upper portion; or (3) by use of asensitizer in the lower part of the structure which is less resistant tothe developer in the case of positive resist.

However, as previously mentioned, if the second layer of an opticalresist is directly applied on the first layer prior to exposure, thelayers dissolve and mix with each other. If the first layer is baked athigh temperature to avoid this dissolution, the first layer hardens andbecomes insoluble in the developer.

One successful lift-off process using standard optical-type resists isdescribed in the patent issued in the names of Franco et al, U.S. Pat.No. 4,004,044 and entitled "Method for Forming Pattern Films Utilizing aTransparent Lift-Off Mask". The method comprises depositing an organicpolymer masking layer, such as AZ-1350 type resist, atop a substrate.After baking to improve adhesion and thermal stability, a separate glassresin layer is spun on over the resist. A second masking layer which mayalso be AZ-1350 is then spun on the resist layer. The second maskinglayer is patterned and windows are opened in the glass layer and thefirst masking layer by reactive sputter etching using two gases. Thesecond reactive sputter step is continued until the edges in theopenings through the glass material overhang the edges in the openingsthrough the first AZ-1350 layer, thereby forming a lift-off mask.Although the Franco et al technique is very effective in providingusable lift-off masks, it is complicated by the need for the glass resinlayer, reactive sputter etching and relatively high temperatures. Asimpler process is desirable.

SUMMARY OF THE INVENTION

It is therefore a principal object of our invention to form a compositeresist structure using conventional photoresists.

It is a further object of our invention to provide said resist structurefor an improved lift-off process.

It is another object of our invention to provide said resist structureto yield other selectable resist profiles.

It is yet another object of our invention to provide such a structurewhich is applicable in low-temperature processes.

These and other objects of our invention are achieved with a novelresist film having two layers, one layer being a standard resistmaterial and the other layer being the same or another standard resistmaterial which is saturated with a dilutant.

For a lift-off process, the lower layer is the standard resist and isprepared so as to be more soluble in the resist developer than theupper, saturated layer. When exposed and developed, the upper layeroverhangs the lower layer.

Most photoresist materials comprise three principal parts: a sensitizer,a resin and one or more solvents. Each solvent has a maximum solubilityfor a particular material and several solvents can be used in aphotoresist as a solvent system which offers maximum solubility for thesensitizer and resin. When these solids dissolved in the solvent systemachieve maximum solubility, i.e., saturation, we have found that thisphotoresist does not dissolve an underlying photoresist of the same orsimilar material. However, this type of system is not feasible becauseit is extremely thick, making it difficult to prepare and too viscous tohandle and apply.

We have found that a saturated solution which does not have theabove-mentioned difficulties can be formed by saturating the resistsystem with a material which is a non-solvent for the sensitizer andresin in the photoresist and which is mixable with the solvents in thephotoresist. In the art, such a non-solvent material is called adilutant.

In one preferred embodiment a dilutant such as xylene is mixed with astandard resist solution containing a phenol formaldehyde resin and adiazo oxide sensitizer to the saturation point. When this solution isdeposited atop a layer of the standard pre-baked resist, the layers donot substantially interact.

Our process is particularly effective in the lift-off fabrication ofJosephson thin film devices where low temperature resist processes areimperative. In Josephson devices, fabrication temperatures of greaterthan around 70° C. will damage the oxide layer, which is only 50 Athick. Moreover, the metallization of the device, designed as it is foroperation at cryogenic levels, is also susceptible. Our process allowsone to use resists which can be effectively used at bake temperatures of70° C. or lower.

BRIEF DESCRIPTION OF THE DRAWING

FIGS 1A-1E are diagramatic cross-sectional views of a metallizationprocess using the lift-off technique in which the photoresist layers areprovided in accordance with our invention. The figures also include aflow chart describing the steps.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS 1A-1E illustrate the formation of the composite mask in accordancewith the method of our invention as well as the utilization of thisnovel composite mask for lift-off type purposes. With reference to FIG.1A, an organic polymeric masking layer 21 is formed on substrate 20.Preferably, layer 21 comprises a positive resist such as AZ-1350J typepolymer which is pre-baked to improve adhesion to substrate 20. Thethickness of layer 21 determines the maximum thickness of the functionalfilm that can be lifted off upon removal of layer 21. Typically layer 21is from 10,000 A to 20,000 A thick.

In the fabrication of integrated circuits, substrate 20 may be asemiconductor material or a semiconductor substrate having a surfacelayer of electrically insulative material, such as silicon dioxide.

The preferred organic polymeric masking material AZ-1350J comprises anovolak-type phenol-formaldehyde resin and a diazo oxide photosensitivecompound dissolved in AZ thinner. The result is commercially availablefrom the Shipley Corporation. Other suitable photoresist materialsinclude diazo type photoresist and others as well as negative resistssuch as KTFR available from the Kodak Corporation, and resins such aspolyvinyl cinnamate polymers.

As illustrated in FIG. 1D a layer of saturated resist 22 is spun ontolayer 21. In the case of using AZ-1350J or the like as the underlayer20, it is preferable to also use AZ-1350J or the like as the top layer.The saturating material, i.e. the dilutant, may be either xylene,toluene, chlorobenzene or Freon fluorinated hydrocarbons.

The saturated resist 22 contains a greater amount of the photosensitiveagent or has a greater molecular weight than resist 21. Either propertymakes layer 22 less sensitive to the developer than layer 21.

As illustrated in FIG. 1D, the layer of the saturated resist 22 is spunon to layer 21. Typically the saturated layer is from between 3000 to3500 A thick. The substrate is then pre-baked and the two layers 21 and22 mix slightly. However, because layer 22 is saturated the mixing isnot substantial.

Openings such as opening 23 are produced in layers 22 and 21 byconventional lithographic techniques such as optical or electron beamexposure. Because of the lesser sensitivity to the developer of upperlayer 22 as compared to the lower layer 21 the latter tends to etchfaster in the developer, thereby yielding the lift-off structure withoverhang 25. overhang 25 in layer 22 permits considerable "overetching"of layer 21 to assure that all of the material of layer 21 has beenremoved in the locations of the desired opening 23. Additionally, theoverhang aids in the elimination of "edge tearing" when the thin filmmaterial is lifted off in subsequent steps of the process. The patterndimensions of the functional thin film material to be deposited onsubstrate 20 through openings 23 are determined by the overhang aperturesize in layer 22.

Next, using the lift-off composite structure of FIG. 1C, a functionalmetallic film 24 is deposited over the structure as shown in FIG. 1D.This metallic film may be any metal used for integrated circuitmetallization, for example, aluminum, aluminum-copper alloys,aluminum-copper-silicon alloys, palladium, chrome, platinum, tantalum,etc. Layer 24 may also be an insulator such as silicon dioxide orsilicon nitride.

In the final step of the lift-off process photoresist layer 21 iscompletely removed by immersion into a solvent, such as acetone or J100photoresist solvents for about 15 to 30 minutes which leaves thin filmlayer 24' in the desired pattern configuration as shown in FIG. 1E. Thesolvent selected should be one which dissolves or swells the polymericmaterial of layer 21 without affecting the film 24'. Such solvents alsoinclude NMP, methylethylketone or trichloroethylene. The solvents usedto dissolve the polymeric material may be the same solvents used inapplying the polymeric coating 21.

EXAMPLE 1

A positive resist material is fabricated with phenol formaldehydenovalak resin and a diazo ketone sensitizer, which is commerciallyavailable from the Shipley Company as AZ1350J. The sensitizer isbelieved to be 4'-2'-3'-dihydroxybenzophenone mixed esters of1-oxo-2-diazo-naphthalene-5-sulfonic acid. Other sulfonated esters ofdihydroxybenzophenone could also be used. These solids are dissolved inethyl cellosolve acetate, or AZ thinner which contains 82.6% ethylcellosolve acetate, 8.8% N-butyl-acetate and 8.6% xylene.

This dissolved resist solution is then mixed with a dilutant such asxylene, toluene, chlorobenzene or freon. The dilutant is added to thedissolved resist gradually by mechanical stirring or agitation. At aconcentration of around two parts xylene and one part dissolved resistsolution, sufficient solids including the resist resin and sensitizerprecipitate. The entire mixture is then filtered through sub-micronfilters, with the filtered mixture being termed saturated resist.

In the process for forming the composite layer, the substrate isprecleaned, treated with a surface adhesion promoter such as HMDS, andthe resist material without said dilutant is applied thereon to athickness of around 18,000 A. The resist is prebaked at 70° to 100° C.for ten to fifteen minutes.

Said saturated resist is then spun on the original resist to a thicknessof around 1200 A as layer 22. The prebaking step is repeated, wherebythe composite resist structure appears as shown in FIG. 1B ready foruse.

After this second pre-baking step the thickness of layer 22 is increasedfrom around 1200 A to around 3000 A by its interaction with layer 21.However, this has no significant effect on the further processingbecause of the thickness of layer 21. Application of AZ1350J typicallyyields a resist layer which is around 18,000 A thick. Even with thedilution of 3 parts AZ1350J with 1 part AZ therein, layer 21 is stillaround 12,000 A thick.

In the case where the liftoff structure shown in FIG. 1C is desired,layer 21 is formulated to be faster dissolving in resist developer thanlayer 22. This is best accomplished by formulating resist 21 to have alower molecular weight than resist 22. The resist described in U.S. Pat.No. 3,666,473, issued in the names of Colom and Levine, is suitable aslayer 21. Alternatively, the concentration of sensitizer in layer 21, ascompared to that in layer 22, may be reduced. This is best accomplishedby using standard AZ1350J resist as layer 21 and adding a quantity ofsaid sensitizer to another portion of AZ1350J for fabricating saidsaturated resist as layer 22. For example, 20 gm. sensitizer isdissolved in 80 gm solvent. This solution is added to 20 ml. AZ1350J sothat a resist solution is formed which is then saturated with xylene.This is applied as layer 22.

Layer 22 is masked so as to define the desired pattern of openings andis exposed to an actinic light source of between 2000-4000 A. Theexposed areas are then dissolved in an alkaline developer, viz. AZdeveloper, to yield the structure shown in FIG. 1C.

EXAMPLE 2

By adding four parts xylene to one part dissolved resist solution, moresolids are caused to precipitate. The resulting saturated resist isthinner than the 2:1 saturated resist in Example 1. When applied atopthe standard resist, the thickness of the saturated resist layer isreduced to around 600 A; in addition, there is less interaction betweenthe two resist layers.

EXAMPLE 3

The application of more than one coat of saturated resist is beneficialin controlling the resist profile obtained after exposure. For example,a thin saturated resist layer fabricated as in Example 2 may be appliedfirst atop the standard resist layer 21. This is followed by one or morecoats of the saturated resists fabricated as in Example 1 or 2 to yielda saturated resist composite layer of a desired thickness.

EXAMPLE 4

A negative resist material is fabricated with a resin ofpoly-cis-isoprene cyclized rubber and a sensitizer which is 2,6-bis(p-azidobenzylidene)-4-methylcyclohexane. These solids are dissolved inxylene.

A portion of the dissolved resist solution is then mixed with a dilutantof Freon which is carried in cellosolve acetate in a ratio of 1:1. Thisis added to the dissolved resist gradually by mechanical stirring oragitation. At a concentration of around one part dilutant-cellosolveacetate solution and one part dissolved resist, solids including theresist resin and sensitizer precipitate. The entire mixture is thenfiltered through sub-micron filters, with the filtered mixture beingsaturated resist.

In the process for forming the composite layer, the substrate ispre-cleaned, treated with a surface adhesion promoter such as HMDS, andthe remaining portion of the original resist material is appliedthereon. The resist is prebaked at 80° to 100° C. for ten to fifteenminutes. Said saturated resist is then spun onto atop the unsaturatedresist.

The prebaking step is repeated, whereby the composite resist structureis ready for use as a resist mask.

The composite is masked so as to define the desired negative pattern ofopenings and is exposed to an actinic light source of between 2000-4000A. The unexposed areas are then dissolved in xylene developer to yieldthe patterned mask.

In conclusion, our invention provides a means for forming a mask of aplurality of layers of photoresist without significant interaction amongthe plural layers. Various patterned resist profiles are therebyachievable by providing for a slower dissolving rate in resist developerin selected resist layers of the composite. in particular, the inventionyields a desirable profile which is undercut to aid in establishing adiscontinuity between the portions of added layers which are applied tothe upper surfaces of a resist and the portions of the layer which aredirectly attached to the substrate. This result is obtained by thefaster dissolution of the underlying resist layer in the developer.Because our process is applicable to the types of resist which do notrequire high temperature baking, our invention is very effective in theformation of devices which require low temperature processes, such asJosephson type devices. This is discussed merely by way of example andis not intended to limit our invention. Clearly, it is also applicableto the formation of standard integrated circuit structures as well as tomore advanced technologies such as bubble memories.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand detail may be made without departing from the spirit and scope ofthe invention.

I claim:
 1. In a resist structure comprising:a first layer of resistmaterial disposed on a substrate; which layer has been prebaked attemperatures not above about 100° C.; a second layer of resist materialdisposed atop said first layer, each resist material being a diazo oxidesensitized organic polymer capable of forming a patterned resist mask toprotect the underlying portions of said substrate; the improvementwherein; said second layer is formed from a resist material solutionwhich is saturated with a dilutant which does not dissolve the firstresist material such that the layers are not substantially mixed.
 2. Theresist structure as in claim 1 wherein said resists include a phenolformaldehyde resin.
 3. The resist structure as in claim 2 wherein saidresists include cellosolve solvent.
 4. The resist structure as in claim3 wherein said dilutant is selected from the group consisting of xylene,toluene, chlorobenzene and freon.
 5. The resist structure as in claim 1wherein one of said layers has a lower solubility rate in resistdeveloper than the other of said layers.
 6. The resist structure as inclaim 5 wherein said one layer has a higher molecular weight than saidother layer.
 7. The resist structure as in claim 5 wherein said onelayer contains a greater proportion of sensitizer than said other layer.8. The resist structure as in claim 5 for a lift-off structure whereinsaid layer which has said lower solubility rate is said second layer. 9.The resist structure as in claim 8 wherein said first layer has athickness of greater than about 10,000 A and said second layer has athickness of greater than about 3,000 A.
 10. The resist structure as inclaim 1 further comprising:a third layer of resist disposed atop saidsecond layer, said third layer being saturated with said dilutant. 11.The resist structure as in claim 1 wherein said resists include apoly-cis-isoprene cyclized rubber.
 12. The resist structure as in claim11 wherein said resists include xylene solvent.